Drive circuit for minimizing power consumption in inductive load



Uiiited 8- ates Patent [72] Inventors T. O. Paine Administrator of the National Aeronautics and Space Administration with respect to an invention of William E. Crawford, Altadena, Calif.

[2]] Appl. No. 851,394

[22] Filed Aug. 19, 1969 [45] Patented Dec. 22, 1970 [54] DRIVE CIRCUIT FOR MINIMIZING POWER CONSUMPTION IN INDUCTIVE LOAD 16 Claims, 3 Drawing Figs.

[52] U.S.Cl 3l7/148.5, 307/1041317/123 [51] 1nt.Cl. ..II01h 47/32 [50] Field ot'Search 317/123, 148.5, 33; 317/123CD; 320/31, 39; 307/104 [56] References Cited 9 UNITED STATES PATENTS 3,125,715 3/1964 Brooks 317/33 Primary Examiner-James D. Trammell Assistant Examiner-Ulysses Weldon Attorneys-J. I-l. Warden, Paul F. Mc Caul and G. T. Mc Coy ABSTRACT: A circuit for driving an inductive load, such as a solenoid, so as to minimize the solenoid power consumption. The circuit parameters are selected so as to apply a driving voltage to the solenoid until the solenoid current exceeds the pull-in current. Then the circuit automatically terminates the driving voltage and the current through the solenoid is permitted to decay to a value just exceeding the drop-out" current. The circuit then continues to cycle on and off to alternately drive current through the solenoid and permit it to decay while always maintaining the solenoid current in excess of the drop-out current but considerably below the pull-in current. This cycling continues until the solenoid activate switch is opened.

SWITCH \VOLTAGE COMPARATOR u o L SOLENOID assessii DRIVECIRCUHFOR'MINIMIZINGPOWER CONSUMPTION IN INDUCTIVE LOAD ORIGINOFTHE INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of I958, Public Law 85-5 68'(72 Stat. 435;42' USC 2457).

BACKGROUND OF THE-INVENT ION 1. Field of the Invention This invention relates generally to circuits for driving in'ductive loads, suchas solenoids, and more particularly; to such'a circuit which minimizes solenoid power consumption while maintaining the solenoid in an activated state.

2. Description of the Prior Art Inaccordance with'a significantfeatureof the preferred em- *bodiment of the invention, means are "provided to initially establisha first high" threshold to thus permit the current level through the-solenoid to'cbuild up tothe pull-incurrent level.

5 Thereafter, a second lower' threshold is ,.automatically snissorsscnmiou'or'rns'bshwiuos FIG. l'is aschematicdiagram illustrating a preferred em- "bodimentof the invention;

FIG.?'2 is a waveform chart illustrating thevoltageeat the upper terminalof the solenoid of FIG. 1; and

IFIG. '3 isa waveform chart illustrating the. current through There are many applications, of course, in which avalve or h l oid,

other mechanism is actuated in response to the energizationof a solenoid. For example only, many valves are solenoid controlled such that the valve is opened' (or closed) when the solenoid is energized or closed (or opened) when the solenoid is not energized. in such arrangements, in order to maintain the valve openedfor a certain interval, it is thenormalipractice to drive current through the solenoid'until the pull-in current level is exceeded and the valve opens and thento maintain this current level for the'full interval; In many applications, where power consumption is not critical, this constitutes a very satisfactory technique for operating the solenoid.

However, in certain applications where power consumption is a significant factor, it is inefficient to maintain the pull-in current level through the solenoid for the entire interval.

OBJECTS AND SUMMARY THE INVENTION In view of the foregoing, it is an object of thepresent invention to provide a circuituseful for energizingasolenoid.and for minimizing the power consumption therein.

It is a more particular'object of the present invention to provide a drive circuit for maintaining the current through asolenoid at a level in excess of a drop-outcu'rrent level but considerably below a pull-in current level.

Briefly, a drive circuit is provided in accordancewithlthe present invention for applying a drivesvoltage' to a solenoid until the solenoid current exceeds a pull-in current level. Thereafter, the circuit automatically terminates the drivevoltage and the current through the solenoid is permitted to decay to a value just exceeding a drop-out. current levell The'circu'it then begins to cycle, alternately supplying -current to" thesolenoid and permitting the solenoid current to; decay, while always maintaining the solenoid current in excesscf the dropout current level and considerably below the'pull-in current level.

In a preferred embodiment of the invention, the solenoid is connected in a series path between 'a transistor switchand a resistor with the path being connected across'a source'of potential. Voltage comparison means are provided for monitoring the voltage across the series resistor which is, of course,

indicative of the current level through the solenoid. The voltage comparison means functions initially to compare the voltage across the series resistor with a first threshold level related to the pull-in current level and subsequently to compare it with a second threshold level related to the drop-out current level. If at any time the series resistor voltage is less than the threshold established at that time, the voltage comparison means closes the series transistor switch to supply more current to the solenoid. When the voltage across the series'resistor then exceeds the threshold, the voltage comparison means opens the series switch to permit the solenoid current to decay. The solenoid current range between opening and closing of the series switch is determined by a positive feedback resistor. Thus, the circuit continues to cycle as long as the potential source remains connected across the series path.

'DE SCRIPTION0F TI-IEPR'EFERREDLEMBODIMENTS Attention is nowcalled: to FIG. lot the drawing which illustrates a preferred drive circuit .embodimentin accordance with. the invention forsupplyingcurrent to an inductive load 1 device such as the'solenoid lt). As is well known, it is normally *necessaryto provide a higher level pull-in current'to a solenoid, e.'g., in orderto actuate a-valve. mechanism, than it is to :maintain the valve mechanism actuated. That is, once the pullin. current level throughthe solenoid has'been exceeded, it is only necessary to maintainithe solenoidcurrent in excess ofa drop-rout level in order to maintain the actuation of the. "mechanism controlled bythe solenoid. The drive'circuit of FIGJl-operates to initially establi'sh'a current through the The. circuit of FIG.' 1' includes a first series path including a :resistor 12 connected in series with the. solenoid 10. In: addition, the emitter collector path of a first transistor switchQl is connected in series with the solenoid l0 and resistor 12. A .voltage is applied across the series path including the-solenoid 10:byclosureofasolenoid activate switch 14. More particu- 'larly, the single-pole,single-throw switch .14 connects the emitter of transistorlQlto aisource of positivepotential 26. The lower terminal of resistor 12 is connected'to ground ter- 'minallo. I

'Unlessotherwise stated, it will be assumed that the solenoid activate -switch 14isxalways :closed. With switch l4 closed, .current w'ill .be conducted through the solenoid 10 if the transistor-switch O1 is forward-biased. Transistor switch Q1 is controlled by transistor Q2 which, as will be seen hereinafter, functions as a voltagecomparator. The emitter of transistor .{QZ is connected to the junction between the solenoid l0 and. resistor .12. The collector .of transistor 02 is connected through resistor 20 to the base of. transistor Q1. The transistor .02 functions as a voltage comparator to com:

"pare the voltageat the junction between solenoid 10 and re sistor i2 with"a"threshold voltage applied to the base of transistor 02. As will'become more apparent hereinaftena first higher threshold level is initially applied to the base of transistor'Q2 to permit the current through the solenoid 10 to build up to the higher pull-in current level. After the pull-in,

current level has been exceeded,- a second lower threshold level isapplied to the base of transistor Q2.

The components for establishing the threshold levels at the base of transistor Q2 includes a capacitor 22 whose upper terminal isconnected to switch 14 and whose lower terminal is connected through a diode 24 to a junction 26 formed between a first voltage divider resistor 28 and a second voltage divider resistor 30. Note that the voltage divider resistors 28. and 30 are connected betweenground terminal 16 and the switch 14. Junction 26 is connected through resistor 32 to the base of transistor Q2. The base of transistor 02 is connected to ground through a zener diode 34.

In addition to the foregoing components, the base of transistor 02 is connected through'a positive feedback resistor 36 to the junction between the collector of transistor switch Qi and the solenoid l0. Diode 38 connects the lower terminal of capacitor 22 to this junction which, for convenience, will hereinafter be referred to as point A. Diode 40 connects the lower terminal of capacitor 22 to ground terminal 16.

In considering the operation of the circuit of FIG. I, initially assume that the solenoid activate switch 14 is open and that the capacitor 22 is completely discharged. When the solenoid activate switch 14 is then closed, a large current will initially flow through capacitor 22, diode 24, and resistor 28 to thus increase the potential on the base of the NPN transistor Q2. The zener diode 34 will clamp the potential rise on the base of transistor 02 to a certain level which will be assumed herein to be +6.6 volts. In any event, this current provided to the base of transistor 02 will forward bias transistor 02 and in turn forward bias transistor O1 to thus provide current flow through solenoid l and resistor 12. As the solenoid current level increases, the voltage across resistor 12 will also increase to thus raise the potential on the emitter of transistor Q2. When the potential on the emitter of Q2 closely approaches the first threshold level of +6.6 volts established by the zener diode 34 on the base of transistor 02, transistor Q2 will cut off and in turn cut off transistor Q1. With transistor 01 cut off, the +26 volts driving voltage will no longer produce a current flow through solenoid 10. However, as is the case with all inductive loads, the current therethrough cannot be immediately terminated. Thus, the energy stored in the solenoid 10 produces a current around the loop including resistor 12, zener diode 34, and resistor 36. As should be apparent, this forces the potential at point A to a slightly negative value equal to the sum of the forward drop across zener diode 34 and the drop across resistor 36. As a consequence of the potential on point A going negative, capacitor 22 will further charge to thereby establish a level thereacross in excess of the +26 volts provided by the power supply. This additional charging of the capacitor 22 effectively biases the capacitor 22 out of the circuit so that is thereafter no longer has any influence, that is at least until switch 14 is opened and again closed to start another operation.

With capacitor 22 effectively out of the circuit, the voltage divider comprised of resistors 28 and 30 will establish the potential on junction 26 which thus constitutes a second threshold level lower than the level initially established by the circuit path through capacitor 22. As the current provided by the energy stored in solenoid l0 decays, the potential at the emitter of transistor 02 will also decrease. When it falls below the level established at the base of transistor Q2 by the resistors 28 and 30, transistor Q2 will become forward biased and thus will close transistor switch Q] to again supply current to the solenoid 10. As the current through the solenoid 10 increases, the voltage across the resistor 12 increases to the point of again cutting off the transistor Q2. The width of the current level range between the upper and lower current levels through the resistor 12 which respectively produce conduction and cutoff in the transistor O2 is determined by the value of the positive feedback resistor 36. When transistor 01 is conducting, resistor 36 feeds back a portion of the +26 volt level at point A to the summation point at the base of transistor Q2. When transistor Q] is not conducting, resistor 36 feeds back a portion 'of a slightly negative potential at point A to the base of transistor 02.

In view of theforegoing, it should be apparent that the circuit will continue to cycle with the transistor switch 01 being closed each time the voltage across resistor 12 decreases to a level below the threshold established at the base of transistor 02 by voltage divider resistors 28 and 30 and feedback resistor 36. When transistor switch 01 closes, the current through solenoid l0 and resistor 12 will then increase to soon thereafter cut transistor Q2 off. This cycling will continue for as long as switch 14 remains closed, When switch 14 is opened, the capacitor 22 isdischarged through the solenoid 10, resistor 12 and diode 40.

The waveform chart of FIG. 2 illustrates the voltage at point A and the waveform chart of FIG. 3 illustrates the current through the solenoid l0. Assume that switch 14 is closed at time t0. As has been explained, the current through the solenoid from the capacitor 22 charges quickly until time :1. The small valley 50 in the current curve represents a change of inductance through the solenoid as the armature thereof is pulled in. At :1, the voltage across the resistor 12 is built up sufficiently to cut 011' the voltage comparator transistor 02 to thereby drop the voltage at point A to some negative value. When the current decreases to level illustrated at time :2, the voltage comparator A2 will then begin to conduct and close transistor switch Q1 to thereby again force the potential at point A up to approximately 26 volts. The circuit will then continue to cycle as illustrated by FIGS. 2 and 3 until switch 14 is opened at time In when the charge stored in capacitor 22 will be discharged through the solenoid.

The cycle rate is determined by the value of. positive feed back resistor 36. That is, if the value of resistor 36 is small, its influence is great causing a wider current range between conduction and cutoff of transistor Q2 and thus a lower cycling rate. On the other hand, if the value of resistor 36 is very high, it will have little influence and thus the second threshold will be very sharply established almost solely by the voltage divider resistors 28 and 30. In this case, the width of the current level range between conduction and cutoff of transistor Q2 will be very small and thus the cycle rate will be very high.

From the foregoing, it will be recognized that a circuit has been disclosed herein for driving an inductive load, such as a solenoid, and for minimizing power consumption within the load. Power consumption is minimized by permitting the inductive load current to build up to a pull-in current level but to thereafter maintain the current through the inductive load at a level substantially below the pull-in level and always above a drop-out current level. During this latter phase of operation, the circuit cycles between solenoid current charging and decaying states. Although the waveforms of FIGS. 2

and 3 illustrate an exemplary circuit operation in which the I duty cycle, i.e., percentage of the total time that the drive voltage is supplying current to the solenoid, is in the order of 3 3 96 percent, embodiments of the invention can operate with as great as a 10:1 power saving. Table I set forth hereinafter illustrates component values utilized in a typical embodiment of the present invention:

TABLE I Capacitor 22 microfarads -2 Resistor 28 ohms- 5k Resistor 30 -do- 50k Resistor 32 do- -10k Resistor 36 do- 500k Resistor 20 do- 10k Resistor l2 do 100 Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art and, consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

I claim:

1. A drive circuit useful in combination with an inductive load for supplying current thereto, said drive circuit comprismg:

a switch;

a resistor,

means for connecting said switch and said resistor in series with said inductive load across a source of potential; voltage comparison meansfor comparing the voltage across said resistor with a threshold voltage signal;

means responsive to said voltage comparison means indicating said voltage across said resistor is less than said threshold voltage signal for closing said switch and to said voltage comparison means indicating said voltage across said resistor is greater than said threshold voltage signal for opening said switch;

means for initially establishing said. threshold voltage signal at a first level; and

' means responsive to said switch opening for establishing said threshold voltage signal at a second level lower than said first level.

2. The drive circuit of claim 1 wherein said switch comprises a first transistor having a base, a collector, and an emitter; and wherein said means for connecting said switch includes means for connecting the emitter-collector path thereof in series with said resistor and inductive load.

3. The drive circuit of claim 2 wherein said voltage comparison means includes a second transistor having a base, a collector, and an emitter:

means coupling said second transistor emitter to said resistor; v means coupling said second transistor collector to said first transistor base; and

means for applying said threshold voltage signal to said second transistor base.

4. The drive circuit of claim 1 including current decay path means for closing a conductive path solely around said inductive load and said resistor.

5. The drive circuit of claim 1 wherein said means for establishing said threshold voltage at a second level includes feedback means responsive to the state of said switch.

6. The drive circuit of claim 1 wherein said means for initially establishing said threshold voltage signal includes circuit path means connected across a source of potential and comprised of a capacitor connected in series with a first voltage divider resistor for establishing said threshold voltage at the junction therebetween.

7. The drive circuit of claim 6 including a second voltage divider resistor connected in parallel with said capacitor; and wherein said means responsive to said switch opening includes means for charging said capacitor to store a voltage thereacross in excess of that supplied by said potential source.

8. The drive circuit of claim 7 wherein said switch comprises a first transistor having a base, a collector, and an emitter; and wherein said means for connecting said switch includes means for connecting the emitter-collector path thereof in series with said resistor and inductive load.

9. The drive circuit of claim 8 wherein said voltage comparison means includes a second transistor having a base, a

collector, and an emitter;

means coupling said second transistor emitter to said resistor; means coupling said second transistor collector to said first transistor base; and means for connecting said junction between said capacitor and said first voltage divider resistor to said second transistor base. 10. The drive circuit of claim 9 including means for clamping the potential applied to the base of said second transistor.

ll. in combination with a solenoid, a drive circuit for supplying current thereto up to a pull-in" level and for thereafter maintaining a current therethrough just in excess of a dropout" level lower than said pull-in level, said drive circuit comprising:

means for connecting a voltage source across said solenoid; means for monitoring the current level through said solenoid and for initially comparing it with a first threshold corresponding to said pull-in" level; means responsive to said current level through said solenoid exceeding said first threshold for disconnecting said voltage source from said solenoid; means responsive to the current level through said solenoid decaying to a level just in excess of said drop-out level for connecting said voltage source across said solenoid; said means for monitoring including means for subsequently comparing the current level through said solenoid with a second threshold lower than said first threshold; and

means responsive to said current level through said solenoid exceeding said second threshold for disconnecting said voltage source from said solenoid.

12. The combination of claim 11 wherein said means for connecting said voltage source across said solenoid includes a first transistor having a base, a collector, and an emitter; and means for connecting the emitter-collector path of said first transistor in series with said solenoid.

13. The combination of claim 11 wherein said monitoring means includes a resistor connected in series with said solenoid and voltage comparator means responsive to the voltage across said resistor.

14. The combination of claim 11 including means for initially establishing said first threshold including circuit path means connected across a source of potential and comprised of a capacitor connected in series with a first voltage divider resistor for establishing said first threshold at the junction therebetween.

15. The combination of claim 14 including means for subsequently establishing said second threshold at said junction including means for charging said capacitor to store a voltage thereacross in excess of that supplied by said potential source.

16. The combination of claim 15 including a feedback resistor coupling one terminal of said solenoid to said junction. 

